The invention relates generally to the field of disc drives and more specifically to a method and apparatus for eliminating repeatable runout introduced during the servo track writing process and media imperfections in a disc drive.
In general, disc drive storage systems record and reproduce information on a recording media. The media generally takes the form of a circular information storage disc having a plurality of concentric storage tracks. A disc drive system generally includes one or more information storage discs capable of magnetically storing information. The discs are rotated by a spindle motor mounted at the center of the disc or discs. The information is read and written to specific locations on the disc(s) using a magnetic (or optical) transducer, commonly known as a read/write head, that flies above the disc surface. Each head is carried over the media by an elongated flexure arm. The flexure arms can be vertically aligned and each is attached to a common head positioning assembly.
The head positioning assembly may be either rotationally mounted, or may take the form of a linear carriage that is free to move back and forth along a single axis. In rotary mounted head positioner assemblies, a voice coil motor rotates the head positioner assembly about a pivot mechanism to precisely position the head(s) relative to the recording media.
The precise positioning of the heads relative to the concentric tracks is typically accomplished by incorporating a closed-loop, electro-mechanical servo system. The implementation of the servo system may include a dedicated servo surface which is associated with one of the plurality of heads in the disc drive system. Alternatively, short bursts of pre-recorded servo data, referred to as a servo burst field, may be written amid the contents of the user data tracks. A servo track writer is employed to write the servo data and a control system monitors the servo surface or the servo burst field data to maintain the precise position of the heads relative to the concentric tracks of the disc(s). Various errors may be introduced during the servo track writing process that may cause track misregistration during the operation of the disc drive.
A significant error introduced by the servo track writer is track squeeze. Track squeeze occurs when the distance of two adjacent tracks written by the servo track writer is smaller than the specified track spacing at certain points. Vibrations during the servo track writing process can cause track squeeze. Track squeeze has to be accounted for as an uncertainty when specifying the track spacing of a disc drive, and therefore, track squeeze limits the maximum achievable track density.
In disc drives, a significant contributor to track misregistration is the repeatable runout written in by the servo track writer (SWRRO). Track misregistration can also be caused by media imperfections. Slight differences of the magnetic properties of the media over the disc surface may cause variations in the magnitude of the servo bursts read by the head. This, in turn, results in a position measurement error and track misregistration.
In FIG. 1, solid line 51 represents an ideal servo track. Dashed line 52 represents the track center after the servo write process. Because of various disturbances occurring during the servo write process and media imperfections, the track center is not smooth. A disc drive actuator typically would have difficulty following this path.
During the operation of the disc drive, a position measurement signal is generated at each servo burst, and fed into a control system. The control system computes a position error signal, which is equivalent to the deviation of the measured actuator position from the desired position. During track following, the position error signal is a direct measure of the track misregistration and includes repeatable and non-repeatable components. The repeatable component, referred to as the repeatable position error signal, includes the repeatable runout written in by the servo track writer (SWRRO), and the disturbance caused by media imperfections. The control system makes use of the position error signal to reposition the head.
If the non-repeatable position error component is neglected, the perfectly circular track center can be followed with zero actuator acceleration. When zero actuator acceleration is achieved (a zero acceleration path or ZAP), track squeeze and track misregistration may be significantly reduced. A basic principle of ZAP correction method is to subtract an appropriate correction factor from the position measurement signal at each servo sample. If the correction factors are determined appropriately, the original zigzag path becomes smooth, i.e. the track center becomes a perfect circle.
Any improvement achieved by subtracting correction factors from the position measurement signal is dependent on how accurately the correction factors are determined. Several procedures have been proposed to compute correction factor values. Most of these methods use inaccurate actuator models, and/or frequency domain computations. Thus, the calculations required by these techniques are complicated, and require many revolutions (usually more than 60) for each track.
The present invention provides a solution to this and other problems, and offers other advantages over the prior art.
In one aspect, the invention provides a method of correcting track misregistration in a servo system for a disc drive including one or more discs includes positioning a head over a track located on a disc and maintaining the head centered over the track. The step of maintaining includes measuring the radial position of the head relative to the disc and determining correction factors for a zero acceleration path. Correction factors are determined by modeling an actuator transfer function to produce an estimated position signal. Subsequently, an estimated disturbance signal is determined by subtracting the measured position and the estimated position signal and filtering it with an adaptation filter. Thereafter, the estimated disturbance signal is subtracted from the measured radial position to produce a modified position measurement signal. The head s repositioned in accordance with the modified position measurement signal.
Aspects of the invention include one or more of the following features. The estimated disturbance signal can be derived using the adaptation filter of Eq. 1:                               w          n                =                              w                          n              -              1                                +                                    v                              n                -                1                                      n                                              (                  Eq          .                      xe2x80x83                    ⁢          1                )            
The raw estimated disturbance (the input of the adaptation filter) can be computed by subtracting an estimated position obtained through an accurate actuator model from the measured actuator radial position.
The disc drive includes a servo loop having a controller and the actuator, where the controller receives as an input the position error signal and outputs an actuator current signal which is coupled to the actuator. The actuator positions the head in accordance with the actuator current signal and outputs a position signal indicative of the radial position of the head relative to the disc. The step of deriving the Plant model includes determining the impulse response of the actuator including deconvolving the actuator current signal from the position signal.
The results of the deconvolving step can be used as coefficients of a finite impulse response filter. The impulse response can be differentiated to reduce the order of the Plant model. The differentiating step can be performed twice. This can include dividing the impulse response by (zxe2x88x921)2 to maintain the integrating property of the Plant model. Any linear trend in the actuator current signal can be removed before feeding it into the Plant model {circumflex over (P)}(z) so as to minimize the effects of non-repeatable error in the position signal.
The step of removing any linear trend can include subtracting any linear trend from the actuator current signal such that a value of the actuator current signal at any sector on the disc is the same as a value at a same sector during a next revolution.
Any mean value of the actuator current signal can be removed before feeding it into the Plant model {circumflex over (P)}(z) to minimize the effects of non-repeatable error in the position signal. Any linear trend can be removed from the estimated position signal before subtracting it from the measured position signal to compute the raw estimated disturbance.
The step of removing any linear trend can include subtracting a linear function from the estimated position signal such that a value of the estimated position signal at any sector on the disc is the same as a value at the same sector during a next revolution.
In another aspect, the invention provides a servo system for a disc drive. The servo system includes a controller and an actuator where the controller receives as an input a position error signal and outputs an actuator current signal to the actuator. The actuator controls a position of a head used in reading and writing data to a disc drive. The actuator includes an actuator output signal that is indicative of the position of the head. The servo system includes means for measuring position information representative of the radial position of the head including a repeatable error component. A processor is included for determining zero acceleration path correction factors. The processor includes an actuator model and disturbance estimator. The actuator model includes the transfer characteristics of the actuator. The disturbance estimator is operable to produce an estimated disturbance signal that is the sum of the position information and the estimated position signal. A means for subtracting the estimated disturbance signal from the position information to produce a modified position error signal is provided.
Advantages of the invention include one or more of the following. A method is provided to determine ZAP correction factors. The computations are easily implementable in a digital signal processor (DSP), and satisfactory accuracy is achieved within 10 revolutions. A system identification procedure is described that does not require any external equipment. The system identification technique results in a low-order discrete transfer function, which accurately matches both the high and low-frequency properties of the actuator.
The actuator model on each individual drive can be identified directly before initiating any ZAP data collection thereby significantly reducing the number of revolutions needed to achieve the desired accuracy. The correction technique simultaneously reduces the inaccuracies caused by modeling errors and non-repeatable disturbances.
These and various other features as well as advantages which characterize the present invention will be apparent upon reading of the following detailed description and review of the associated drawings.